How to execute tasks in ExecutorService sequentially?

Question

I have three threads that are joined, i.e. the second thread executes after the first dies.

This is the code I have:

public class Main {
    public s         


        

Answer 1

You can control sequential execution of threads using SingleThread ExecutorService with future.get() method.

DummyTask Class

import java.util.concurrent.Callable;

public class DummyTask implements Callable
{

int taskId;

public DummyTask(int taskId) {
    this.taskId = taskId;
}

@Override
public Integer call() throws Exception
{
    System.out.println("excuting task... Task Id: " + taskId);
    return taskId;
}

}

SequentialExecution Class

package com.amit.executorservice;

import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;

public class SequentialExecution 
{
public static void main(String[] args) throws InterruptedException, ExecutionException {

    DummyTask task1 = new DummyTask(1);
    DummyTask task2 = new DummyTask(2);
    DummyTask task3 = new DummyTask(3);
    Future result = null;
    ExecutorService executor = Executors.newSingleThreadExecutor();

    result = executor.submit( task1 );
    // future.get() Waits for the task to complete, and then retrieves its result.
    result.get();

    result = executor.submit( task2 );
    // future.get() Waits for the task to complete, and then retrieves its result.
    result.get();

    result = executor.submit( task3 );
    // future.get() Waits for the task to complete, and then retrieves its result.
    result.get();

    executor.shutdown();

}
}

Output

excuting task... Task Id: 1
excuting task... Task Id: 2
excuting task... Task Id: 3

Output will always be same and all task will get executed in sequence.